1. Field of the Invention
The present invention refers to a method of a direct voltage arc discharge enhanced treatment of at least portions of the surface of an object where a gas or gas mixture, respectively hereinbelow termed process gas which is adapted to react at least partly is fed into an evacuated main chamber with the object to be treated and gaseous reaction products are drawn off from the chamber and a direct voltage arc discharge is maintained in the chamber whereby a flow of charged particles is coupled thereinto.
It also refers to a vacuum treatment arrangement with a vacuum chamber, an inlet arrangement opening thereinto for a process gas introduced at least in part to react in the main chamber, a draw-off arrangement for gaseous reaction products, an opening arrangement for introducing a flow of electrically charged particles into the main chamber, and an electrode arrangement for the generation of a direct voltage arc discharge in the chamber.
2. Description of the Prior Art
A method of the kind mentioned above and an arrangement are disclosed in Swiss Specification CH-A-664,768. In addition, U.S. Pat. No. 4,851,254 discloses a method for a reactive plasma enhanced coating of objects, in which a direct voltage arc discharge is maintained in a vacuum chamber between two closely spaced electrodes, which arc discharge operates at a low voltage below 100 V and relatively high arc currents in the range between 50 A and 100 A.
The object to be coated is placed outside of the cathode/anode space in an insulated manner and parallel to the arc discharge. A process gas which reacts at least partly is jetted to the plasma through a pipe conduit located opposite the object to be treated with respect to the anode/cathode space.
In plasma chemical methods the reaction space is substantially limited to a region surrounding the plasma zone of which the extent is of the magnitude of the length of the free path of the gas. The latter value is in turn proportional to the inverse value of the pressure. A lowering of the pressure in order to increase the size of the reaction space is, however, generally not possible, because the concentration of the reactant gases obviously also decreases when the pressure is lowered.
Therefore, this procedure is insofar deficient in that only small surfaces of the object can be coated, due to the small surface area which is adjacent the short anode/cathode space, and due to the point-like size of the jetting-in of the reactive process gas.
A further procedure of a reactive, plasma enhanced coating (PECO) is disclosed in U.S. Pat. No. 4,859,490. The apparatus disclosed therein includes a freely accessible glowing cathode coil, a screen connected relative thereto and maintained at a positive, anodic potential, and an electrode located opposite the screen relative to the glowing cathode and connected cathodically relative to the screen onto which the object to be treated is placed.
Gas, which is predominantly brought to react in the plasma generated between the glowing coil and the screen is jetted centrally into the main chamber and against the glowing coil. The drawback of this procedure is that the reactive gas must first flow through the glowing coil and thus reacts with the glowing coil. The effect thereof is that on the one hand the coating process of the commodity may be disturbed by the coil and on the other hand the useful operating life of the glowing coil is drastically reduced.
An example of such a condition shall be the depositing of diamond coatings from hydrocarbon/hydrogen mixtures. The carburetion of the filaments of refraction metal proceeds thereby for such arrangements at such a high speed that they fail due to breaking after one to three coating cycles. From the viewpoint of a controlled industrial production this is unacceptable. Similar problems are encountered in other gases in form of a scaling or forming of hydrides or silicides.
The drawbacks of the method and apparatus disclosed in U.S. Pat. Nos. 4,851,254 and 4,859,490 are overcome by the procedure according to Swiss Patent No. CH-A-664,768 mentioned above, in that it is known from this document to generate a low voltage direct voltage (d.c.) arc discharge between an anode and a cathode in that electrically charged particles, electrons and ions, are fed into the discharge space by means of which it becomes possible to maintain between anodes and cathodes an arc at low arc voltages, which electrodes are located at a quite larger distance from each other than according to U.S. Pat. No. 4,851,254.
Due to the fact that also in contrast U.S. Pat. No. 4,859,490, for example, the thermally emitted electrons are not generated in the main chamber but rather outside of the main chamber and are coupled into the main chamber through an opening arrangement thereinto, the contamination of the treatment by the material of the hot coil or the reduction of the useful operating life thereof, respectively by the treatment process is also remedied.
The drawback of the procedure according to Swiss Specification CH-A-664,768 is now on the one hand that in the edge area of the treatment chamber a linear jetting-out of process gas with gas which is to be brought to react is made, or that an inhomogeneous discharge or distribution of plasma, occurs in the inner space of the main chamber seen over its volume.
In many methods this is disturbing to such an extent that it prevents the practicing thereof. Every plasma-chemical reaction necessitates a certain plasma density, i.e. a minimal space density of charge carrying particles where it should proceed.
At the arrangement according to Swiss Specification CH-A-664,768 the space density decreases rapidly in the radial direction. An increase of the arc power does not always solve this problem. The heating up of the substrates in the arc plasma is supplied predominantly by convection. The radial extent thereof is unrestricted. An increase of the power causes a substantially larger increase of the thermal loading on the substrates than of the plasma density which is needed for the depositing.